This invention relates to a speed control system for a single phase AC induction motor and, more particularly, to such a system which maintains a desired speed for the motor under varying load conditions.
Single phase AC induction motors are widely used for driving various types of equipment, from small home appliances to refrigerators to climate control system fans. It is often desirable to be able to control the speed of the motor and maintain a desired speed under varying load conditions. It is therefore a primary object of the present invention to provide a speed control system for a single phase AC induction motor.
Different types of motor speed control systems are presently available. One such type is the inverter type which varies the frequency of the electrical power applied to the motor to control its speed. This is accomplished by an intermediate step of converting the incoming AC power into DC power and then providing a square AC waveform at the desired frequency. Such a system has among its disadvantages the fact that it is relatively expensive to implement and further, that the square AC waveforms produce more heat and noise than conventional sinusoidal AC waveforms. Another common speed control system utilizes the principle of voltage reduction, which lowers the voltage to the motor and allows the rotor to slip under its synchronous speed. While inexpensive, such a system produces waste heat in the motor as well as excessive motor noise or hum. Additionally, the motor can stall in response to a small change in the load. It is therefore a further object of the present invention to provide a motor speed control system which does not suffer from any of the aforementioned disadvantages.